Gelatos are water-soluble and dry permanently on a porous surface. For that reason, they are not permanent on vellum unless they are blended into a medium such as gel, glaze or gesso. However, as this creates a "wet" medium, it will cause vellum to curl.

We suggest using Pitt Artist Pen Big Brush on vellum instead - they work excellent, are transparent on vellum and are permanent when fully dry.